An aqueous inkjet ink is generally composed of a pigment, a pigment-dispersing resin, water, a hydrophilic solvent (a humectant, a penetrating agent), and a binder resin. Among these, the binder resin is used for the purpose of enhancing the coating film durability of printed materials. In the case of inkjet, it is also necessary to take the ejection property of the ink into consideration at the same time. In many cases, resin fine particles that have low viscosity even at high solids content are used as the binder to a large extent. In order to maintain the ejection stability or storage stability of the ink, and also to promote an enhancement of the coating film durability of printed materials, various resin fine particles have been reported.
Patent Literature 1 discloses an aqueous inkjet ink composition which has a charge of the same polarity as that of a colorant, characterized in that the glass transition temperature (Tg) of the resin emulsion is 35° C. or higher, and the minimum film-forming temperature (MFT) is 20° C. or lower. However, the stability of the resin fine particles against the hydrophilic solvent that is included in the ink solvent is not fully taken into consideration, and there is a problem with the ejection stability or storage stability of the ink composition.
Patent Literature 2 discloses an aqueous inkjet ink composition characterized in that the glass transition temperature (Tg) of the resin fine particles is 40° C. to 80° C., and the ink composition contains a water-soluble surface agent selected from the group consisting of monoalkyl ethers of alkylene glycols, 2-pyrrole, N-methylpyrrolidone, and sulfolane. For this ink composition as well, the resin composition is not taken into consideration, and it is difficult to realize excellent ejection stability of the ink composition, based only on the glass transition temperature of the resin and the solvent composition. Furthermore, it is also inappropriate to say that the ink composition has satisfactory storage stability.
Patent Literature 3 discloses an aqueous inkjet ink composition characterized by including sulfonic acid group-containing resin fine particles, an acetylene glycol-based penetration enhancer, and triethylene glycol monobutyl ether. However, it is not feasible to achieve sufficiently both the storage stability of the ink composition and the coating film durability of printed materials, only by the fact that the resin fine particles have sulfonic acid groups.
There has been hitherto a demand for an aqueous inkjet ink composition which is excellent in the storage stability and ejectability of the ink, printing suitability of printed materials, gloss, brightness, and coating film properties (adhesiveness, scratch resistance, water resistance, and solvent resistance).